1. Field of the Invention
The present invention relates to a protective agent block for use in a protective agent applicator, a method of manufacturing the protective agent, a process cartridge, and an image forming apparatus.
2. Description of the Background
In the image forming apparatus employing electrophotography, images are formed on recording media, etc. by processes of charging, irradiation, development, transfer etc. applied to an image bearing member. A minute amount of corona products produced in the charging process and/or residual toner remaining after the transfer process are attached to the surface of the image bearing member until they are removed by the cleaning process after the transfer process. Thereafter, the image forming starting from the charging process is conducted again on the image bearing member.
A cleaning system having a rubber blade is typically used in the cleaning process because such a rubber blade has a simple and cost-saving mechanism with a good cleaning performance. However, since the rubber blade is pressed against the image bearing member to remove the attached material on the surface thereof, the rubber blade and the image bearing member are under a large mechanical stress caused by friction between the surface of the image bearing member and the rubber blade. This easily leads to attrition of the rubber blade and the surface layer of the image bearing member, which shortens the working lives of the rubber blade and the image bearing member. This attrition is significant particularly on the surface layer of an organic photoconductor.
On the other hand, the toner for use in forming images is reduced in size to deal with the demand for improvement on the image quality. In the case of an image forming apparatus using a toner having a small particle diameter, the ratio of residual toner that slips through the edge portion of the cleaning blade and the surface of the image bearing member tends to increase. This is particularly true when the dimension accuracy and/or assembly accuracy is not sufficient or the cleaning blade partially vibrates, which prevents formation of quality images.
Therefore, reducing the deterioration of the image bearing member and the cleaning blade caused by abrasion and improving the cleaning property of the surface of the image bearing member are demanded to prolong the working life of the image bearing member and maintain the quality of images over a long period of time.
To meet this demand, Japanese Patent Application Publication No. 2004-279788 (JPAP-2004-279788-A) describes a method in which a brush, etc. is pressed against a block of metal soap formed of zinc stearate, etc., to obtain fine powder of the metal soap, which is supplied to an image bearing member to form a film of a lubricant thereon by a cleaning blade.
The metal soap formed of zinc stearate, etc. improves the lubrication property of the surface of the image bearing member and reduces the abrasion between the image bearing member and the cleaning blade. In addition, the cleaning property of the residual toner is improved. Therefore, using such metal soap is extremely preferred.
In addition, color image forming apparatuses have been widely used in recent years, which leads to an increasing demand for the quality of images. Thus, the current dominant charging system uses an AC charging system which applies an AC voltage overlapped with a DC voltage to the surface of the image bearing member using a charging roller as a charger. Furthermore, since the AC charging system using a charging roller satisfies the need for size reduction and produces a less amount of oxidized gases such as ozone and NOx, there is a great need for the AC charging system. However, in the AC charging system, the image bearing member is repetitively charged positively and negatively several hundreds to several thousands of times per second depending on frequency. Thus, the image bearing member is easily and heavily damaged in comparison with a DC charging system, in which an image bearing member is positively charged only once while the image bearing member passes through the charging device. Therefore, damage to the image bearing member in the AC charging system accumulate by far sooner than in the DC charging system so that protecting the image bearing member from the damage is demanded for the AC charging system. The metal soap improves the lubricity of the surface of the image bearing member and receives the hazard by AC charging, thereby protecting the image bearing member. Therefore, a large amount of metal soap tends to be applied.
Metal soap is generally of a block form and supplied to the image bearing member in a form of particulates produced when the metal soap is rubbed by a brush pressed against the metal soap. The metal soap supplied to the image bearing member is crushed by the blade and forms a protection layer of the metal soap.
However, if the brush is strongly pressed against the block of metal soap to increase the application amount thereof, large particles of the metal soap tend to be produced and supplied to the image bearing member. Such large particles easily slip through the cleaning blade and cause non-uniform application on the surface of the image bearing member.
In addition, toner and lubricants such as the metal soap that have slipped through the cleaning blade scatter to and fixate on the charging roller, thereby increasing the electric resistance of the charging roller, resulting in bad charging performance. Therefore, protecting the charging roller from contamination is another issue
Typically, the working life of an image bearing member is prolonged but a charging roller and a cleaning blade tend to be replaced when they deteriorate. However, in terms of concerns for the environment, there is a strong need for prolongation of the working life of each member of the charging roller, the cleaning blade, the image bearing member, etc. Therefore, technologies are demanded to prevent deterioration of and contamination on each member.
In an attempt to deal with the issue that the powder of the metal soap supplied to the image bearing member passes through the cleaning blade and scatters to and fixates on the charging roller, which leads to a bad charging performance, JPAP-2008-134467-A describes a protective agent in which boron nitride is blended in metal soap. In JPAP-2008-134467-A mentioned above, it is reported that scattering of the metal soap to the charging roller and abrasion of the cleaning blade are lessened for an extended period of time by blending boron nitride with metal soap (zinc stearate) so that blending an inorganic lubricant of boron nitride with metal soap is effective. In addition, it is possible to simply replace a block of typically-used zinc stearate with this blended lubricant.
The protective agent block of the metal soap is manufactured by heating the metal soap to a temperature higher than the melting point thereof followed by cooling down. On the other hand, the protective agent in which boron nitride is blended in the metal soap is manufactured by mixing and compacting powder of boron nitride and powder of the metal soap as described in JPAP-2010-26461-A. By using the protective agent block manufactured by this method, quality images can be produced.
However, when the thickness of the protective agent block is increased, for example, 15 mm or more to protect the image bearing member for an extended period of time, thereby reducing the frequency of exchanging the image bearing member, the powder of boron nitride and the powder of the protective agent block tend to be not homogenized in the protective agent block and the protective agent block is deformed in severe cases. Alternatively, even though the form of the protective agent block is normal, defective images having black streaks or image blurring may be produced as image forming is repeated. Such defective images greatly vary depending on the manufacturing lot.
Since the powder of the metal soap and boron nitride are generally significantly different with regard to the particle size and the density, it is difficult to uniformly disperse boron nitride in the protective agent. If the block has a portion containing an extremely small amount of boron nitride, the charging roller is easily contaminated partially, thereby increasing the electric resistance of that portion, where the charging performance deteriorates, resulting in production of defective images having streaks. By contrast, if the block has a portion containing an extremely large amount of boron nitride, boron nitride tends to accumulate on the image bearing member and the metal soap and silica serving as an external additive furthermore accumulate, which increases the possibility of production of defective images.
Moreover, since the metal soap and boron nitride for use in the protective agent block are fine powder, when the powder is placed and molded in a mold (cast), particles of the metal soap and boron nitride tend to leak from gaps in the mold. In addition, since the volume when the metal soap and boron nitride are compacted changes greatly from the volume when the metal soap and boron nitride are placed in the mold, it is necessary to compact the powder while removing air between each particle. However, when the thickness of the compacted protective agent block is too thick, for example 15 mm or more, the air between each particle is not be able to be removed so that the density of the compacted protective agent is locally different. Moreover, when the thickness of the compacted protective agent block is too thick, the rate of the particles of the protective agent that have leaked from the mold tends to increase. Therefore, the rate of the metal soap and boron nitride tends to significantly change locally or the density of the protective agent block tends to change. If the rate of the metal soap and boron nitride is different locally, the amount of the protective agent scraped by the brush pressed thereagainst changes from place to place so that protection of the image bearing member decreases at portions where the amount of the protective agent scraped by the brush is small, resulting in production of defective images having streaks.
In addition, soft portions in the protective agent block are scraped sooner. Since the abrasion speed of the image bearing member is drastically different between when the protective agent is supplied to the image bearing member and when the protective agent is not supplied to the image bearing member, no matter how much hard portions remain in the protective agent block, defective images are produced when a portion where the protective agent has exhausted is formed.
In particular, in an environment in low temperatures, when the density of the protective agent block is not uniform therein, how the protective agent block is scraped and the amount of the protective agent supplied to the image bearing member tend to be locally dependent,
The accuracy of molding is improved by increasing the size of the particles of boron nitride and the metal soap. However, if the protective agent block is manufactured by using boron nitride and the metal soap having large particle diameters, the protective agent scraped by abrasion between the brush and the protective agent block has a large particle diameter. Therefore, application of the protective agent to the surface of the image bearing member tends to be non-uniform and particles of the protective agent that pass through the blade tend to increase in number and attach to the charging roller, resulting in production of defective images with streaks. In addition, boron nitride tends to accumulate on the image bearing member and the metal soap and silica serving as an external additive to the metal soap and toner furthermore accumulate, which creates a problem of production of defective images.